June and September are the best months. The sea is warm, the beaches are less overwhelmed than in July and August, most businesses are open, and ferry connections to Split run frequently. The interior looks its best in early summer when the olive trees and vines are still lush.
July and August are peak season. Zlatni Rat is packed; accommodation prices double or more; and the catamaran from Split to Bol can fill up. The island is still beautiful, but you will share it with everyone else. Book accommodation months ahead.
Spring (April–May) is quiet and very pleasant for walking and cycling. Some restaurants and boat services haven't opened for the season, but the hiking trails are at their best. Vidova Gora offers clear long-range views before summer haze settles in.
Winter (November–March) is deep off-season. Supetar stays open (the ferry keeps running), but Bol is largely closed. A few locals-only konobas remain open. Not recommended unless you want total solitude.
Events: The Bol windsurfing season runs from May to October, with the maestral wind strongest in July and August. There is no major festival on the island, but Split's Diocletian's Palace Summer Festival (nearby on the mainland) draws visitors in July.